Light is known to interact with endogenous or exogenous chemical agents in the skin or eyes, to produce photosensitization (phototoxicity or photoallergy). The objective of this project is to determine whether light-induced free radicals play a role in photosensitization. Electron spin resonance studies have shown that UV-irradiation of the anti-psoriatic drug anthralin (AN) resulted in the generation of the superoxide anion radical (O2.), which was identified by spin trapping with 5,5-dimethyl-1-- pyrroline-N-oxide (DMPO). In the absence of oxygen, the drug abstracted hydrogen atoms from the solvent ethanol. However, 1,8-dihydroxyanthraquinone (1,8-DHAQ), the major AN photoproduct, was much more active than AN itself in generating superoxide and ethanol radicals. This suggests that AN photosensitization may be due to 1,8-DHAQ and not AN. Disperse blue 35 is an anthraquinone-based dye mixture which causes photocontact dermatitis in factory workers. The main component of the dye, 4,5-diamino-1,8-dihydroxyanthraquinone, was found to be a potent generator of both singlet oxygen (1-O-2) and O2. upon visible light irradiation. UV-irradiation of another photosensitizing anthraquinone-derived dye benzanthrone (7H-benz[de]anthracen-7-one) resulted in the generation of both 1-O-2 and O-2.in high yield. Active forms of oxygen were also implicated in the photo-killing of gram-positive bacteria by curcumin. In contrast halogenated aromatic photosensitizers (e.g. amiodarone, bithionol, salicylanilides, chlorpromazine) undergo dehalogenation upon UV-irradiation to yield the corresponding aryl radicals which may undergo hydrogen-abstraction reactions in vivo.